In a semiconductor device such as a metal oxide semiconductor field effect transistor (MOSFET), a source electrode and a drain electrode can be provided on an upper side of a semiconductor substrate in some cases. In this type of semiconductor device, the source electrode is provided over a source region and the drain electrode is formed in parallel with the source electrode. A part of the drain electrode can be provided in the semiconductor substrate.
Techniques for reducing on-state resistance between the source electrode and the drain electrode include a method of removing a high-resistance drift layer from a region in which the drain electrode is formed. In this method, a trench that reaches a drain region is formed in a drift region, for example, with isotropic etching, and the drain electrode is formed in the trench.
However, in the trench formed by the isotropic etching, an inner wall of the trench typically has a curved surface. Accordingly, a semiconductor layer in the vicinity of an opening of the trench may have a sharp eave-like feature at the upper surface of the substrate. When forming the drain electrode in the trench thus formed, a local stress may be applied to the sharp eave-like portion due to a difference between a coefficient of thermal expansion of the drain electrode and a coefficient of thermal expansion of a semiconductor material, or due to an inner stress of the drain electrode. Therefore, the sharp portion may be damaged and/or peeling may occur between the sharp portion and the drain electrode.